l- and d-Proline Adsorption by Chiral Ordered ... - ACS Publications

Apr 4, 2012 - Instituto de Síntesis Química y Homogénea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain. •S Supporting Information...
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L-

and D-Proline Adsorption by Chiral Ordered Mesoporous Silica

Clara Casado,† Joaquín Castán,†,‡ Ismael Gracia,†,‡ Miriam Yus,†,‡ Á lvaro Mayoral,§ Víctor Sebastián,† Pilar López-Ram-de-Viu,‡,∥ Santiago Uriel,‡ and Joaquín Coronas*,† †

Department of Chemical and Environmental Engineering and Instituto de Nanociencia de Aragón (INA), ‡Department of Organic Chemistry, and §Laboratorio de Microscopías Avanzadas, INA, Universidad de Zaragoza, 50018 Zaragoza, Spain ∥ Instituto de Síntesis Química y Homogénea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain S Supporting Information *

ABSTRACT: Chiral ordered mesoporous silica (COMS) was synthesized in the presence of amino acid proline by combining tetraethyl orthosilicate and quaternized aminosilane silica sources. The as-prepared materials were activated by calcination or microwave chemical extraction to remove the organic templates. The powder X-ray diffraction and N2 adsorption characterization revealed in COMS the structural and textural features of MCM-41-type silica. The chirality of the material was disclosed by mixed and separate L- and D-proline adsorption on the COMS prepared with L-proline (L-Pro-COMS) and Dproline (D-Pro-COMS). It was found that the maximum L-proline and D-proline adsorption capacities on L-Pro-COMS were ca. 2.3 and 0.6 mmol/g, respectively, while the adsorption of D-proline was higher than that of L-proline on D-Pro-COMS. Finally, both activation routes yielded enantioselective silicas able to separate proline racemate.

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and also ordered mesoporous materials SBA-1511 and MCM4112 have been used to adsorb various amino acids from water solutions. Amino acids of different polarities have been separated by adjusting the pH of the solution appropriately.7 The adsorption of amino acids on zeolites is in general dominated by electrostatic interactions,10 although hydrophobic interactions involving nonpolar side groups7,9 and steric8 interactions complete the overall molecule−adsorbent picture. In addition to providing information on the separation or purification of amino acids,7 the study of the adsorption of these molecules can give insights into the adsorption of proteins or enzymes on solid materials.13 Moreover, the adsorption of amino acids on minerals is an important step in the concentration of these molecules. Several papers have dealt with this topic14,15 since it is thought to have played a role in the origin of life.16 All the proteinogenic α-amino acids except glycine are chiral molecules, all of them possessing the L-configuration. This

INTRODUCTION α-Amino acids, an important class of organic compounds, containing an amino and a carboxyl group in the same carbon atom (the α-carbon), are critical to life because they constitute the building blocks of proteins and biopolymers carrying out the most diverse functions in organisms. The physiological importance of α-amino acids ensures a sustained interest in their chemistry and properties, particularly in the pharmaceutical exploration for new drugs or products with biological applications. Amino acids are commonly used in food technology, drug synthesis, and cosmetics. Continued research on α-amino acids has also led to their use in diverse areas such as the biodegradable plastics industry,1 drug delivery systems,2 or stereoselective laboratory synthesis.3 There are several routes for the production of amino acids, all involving the use of separation techniques to recover and purify them. Besides chromatographic and electrophoresis methods,4,5 amino acids are commonly separated by organic ion exchange resins.6 In addition to these more classical procedures, adsorption from solution into molecular sieves is becoming increasingly widespread in separation and purification processes. In particular, zeolite β,7,8 ZSM-5,7,9 zeolite Y,10 © 2012 American Chemical Society

Received: February 29, 2012 Published: April 4, 2012 6638

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Flourochem) (Figure S1a, Supporting Information) as the initiator in the presence of L-, D-, or DL-proline (99 wt %, Alfa Aesar). The molar composition was TEOS:C18-TMS:amino acid:H2O:NaOH = 6:1:2:1000:4. Typically, 54 g of a precursor gel containing L-proline (LPro-COMS) was prepared by adding 0.605 g of amino acid, 2.637 g of C18-TMS, and 3.337 g of TEOS as the main Si source to a solution of 0.423 g of NaOH (99 wt %, Aldrich) in 47.58 g of H2O, in that order, and stirred at room temperature to homogeneity. The pH measured then had a value of 10.8−11.0. This value was approximately the same at the end of the synthesis. The resulting mixture was maintained at 80 °C for 24 h. The product was then filtered, washed copiously with distilled water and ethanol, and dried at 80 °C overnight. Around 0.6 g of white powder was recovered after drying. The organic material was removed by calcination at 650 °C for 8 h at a heating rate of 2.5 °C/ min. For comparison purposes, samples were prepared with D-proline (D-Pro-COMS) and DL-proline (DL-Pro-COMS) and without amino acid. For the chemically extracted samples, the microwave (MW) oven employed was a computer-controlled Milestone ETHOS Plus, with pressure and temperature sensors in one of the Teflon autoclaves. The microwave power and acid digestion temperature and duration were kept constant for each experiment at 500 W, 200 °C, and 15 min, respectively. A second extraction cycle was run to study the effectiveness of different digestion solutions. Generally, 100 mg of solid powder was mixed in a Teflon-lined microwave autoclave with 10 mL of digestion solution. These conditions were optimized to preserve the microwave autoclave from mechanical stress, since high pressure was achieved during the digestion procedure. Oxidant−acid solutions were prepared to activate the porosity of COMSs by extracting the organic template: sulfuric acid (H2SO4; 96%, Carlo Erba), nitric acid (HNO3; 68%, Carlo Erba), and mixtures at different volume rates of nitric acid + hydrogen peroxide (H2O2; 35%, Sigma-Aldrich). Characterization. The ordered mesoporous materials obtained were analyzed by powder low-angle X-ray diffraction (LA-XRD) in a Philips X’Pert diffractometer with Bragg−Brentano geometry and Cu Kα (λ = 1.5418 Å, 40 kV, 20 mA) radiation. Following degassing at 200 °C for 10 h under vacuum, N2 adsorption/desorption isotherms were obtained using a Micromeritics Tristar 3000 surface area and porosity analyzer. The specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) method, whereas the Barrett− Joyner−Halenda (BJH) method was used to estimate the pore volume and pore size distribution from the corresponding adsorption branch. Selected samples were deeply crushed using a mortar and pestle, ultrasonically dispersed in acetone, and placed on lacey carbon-coated copper grids. Subsequently, they were observed at 200 kV in a JEOL2000 FXII transmission electron microscope and at 300 kV on a TECNAI F30, point resolution 1.7 nm and Cs 1 mm. Adsorption of Amino Acid. To determine the adsorption of the enantiomer, L or D, of proline amino acid by calcined L- or D-ProCOMS, amino acid solutions with concentrations of 0.5−200 mmol/L were prepared by dissolving the amino acid in deionized water. The pH of the solutions coincided with the isoelectric point (pI) to ensure that amino acid was in its zwitterionic form, which is more convenient for adsorption from aqueous solutions. After 1 week of contact of the constantly stirred solution (5 mL) with the porous material (40 mg) at room temperature, the adsorption amount of amino acid was determined by measuring the change of the amino acid concentration before and after adsorption using a UV−vis spectrophotometer and a thermogravimetric analyzer. In the case of the UV−vis determination, the aqueous solution was removed from the corresponding dispersion using a cellulose acetate 0.20 microfilter (JAC 020 25, Albet Labscience). The measurement was carried out by using a procedure developed for proline determination32 based on the maximum absorbance shown by proline at 519 nm when reacted with ninhydrin in acid media. A typical run was as follows: 10 μL of the microfiltered sample was dissolved in 1 mL of water, to which 1 mL of ninhydrin acidic solution was added. This ninhydrin solution was prepared according to the literature.32,33 An acidic solution was prepared by mixing 60 mL of a 6 M solution of phosphoric acid (85%, Alfa Aesar) and 40 mL of glacial acetic acid

reflects the fact that enzymes responsible for protein synthesis have evolved to recognize and utilize only the L-enantiomers, which proves the importance of chiral recognition and discrimination in biological environments. In this context, the development of chiral materials (in most cases, porous silica supports functionalized with an organic chiral selector) that preferentially interact with one enantiomer has permitted the chromatographic resolution by HPLC using chiral stationary phases (CSPs) of racemates of amino acids and other chiral substances of industrial and analytical interest.17−19 Our contribution to the development of some CSPs of great chemical stability and wide applicability20,21 allowed the enantioseparations of different phenylalanine and other amino acid surrogates on a semipreparative scale.22−24 Even though the challenge of obtaining an enantiopure porous zeolitic framework has already been recognized,25 only very recently has the enantioselective recognition of enantiomers by some natural zeolites (goosecreekite and nabesite) been reported.26 The surface and pore properties of meso- and macroporous materials are essential for specific applications such as adsorption and separation. The removal of the organic template from the inorganic framework is a necessary step to achieve porous structures with remarkable properties. Decomposition (thermally, by UV radiation27 or by microwaves28) and extraction (by conventional solvent or by supercritical fluid29) are two general methods for removing the organic template. Nevertheless, the three most important requirements for the template−framework separation, i.e., (1) complete removal of the template, (2) minimum operation time, and (3) ideal structural and surface properties of the inorganic framework, cannot be simultaneously achieved by any of the abovementioned methods. Removing the organic template by microwave wet acid digestion has shown marked benefits over the other methods, such as greater speed, lower structural shrinkage, larger surface areas and pore volumes, and retention of a higher degree of silanol groups.30 These advantages have two possible explanations: (a) the energy transfer is greatly improved and (b) the microwave field has a specific chemical influence on organic molecules.31 Chiral ordered mesoporous silica materials have been developed in our laboratory.12 These materials possess the 2D hexagonal structure and properties of MCM-41 materials, i.e., narrow pore size distribution, large surface area, tunable pore size, and modifiable surface properties, and show, additionally, selectivity in the separation of several racemic mixtures after calcination. In this work, the performance of the so-called calcined L-Pro-COMS (COMS = chiral ordered mesoporous silica), obtained in the presence of amino acid Lproline, on the adsorption of L- and D-proline is presented. The activation of the L-Pro-COMS materials by chemical extraction using microwaves is also explored to elucidate if chiral properties are sensitive to the activation method. Some experiments with DL-Pro-COMS and D-Pro-COMS, prepared in the presence of racemic proline and D-proline, respectively, have also been performed for comparison purposes.

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EXPERIMENTAL SECTION

Synthesis of COMS. COMS powders were prepared by a procedure developed in our laboratory12 using tetraethyl orthosilicate (TEOS; 98 wt.%, Aldrich) as the main Si source and the quaternized silicon source N-3-[3-(trimethoxysilyl)propyl]-N-octadecyl-N,N-dimethylammonium chloride (C 18 -TMS; 50 wt % in methanol, 6639

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(99%, Cofarcas). Ninhydrin (200 mg) (puriss. p.a., Aldrich) was dissolved in 15 mL of this acid solution. The samples were stirred in a water bath at 100 °C for 30 min to let the reaction between proline and ninhydrin acid take place to give the colored product. Then the solution was cooled by being kept for 10 min in an ice bath. Toluene (6 mL) was added to the samples, and the resulting solution was stirred for 2 min. After 1 h at room temperature, two phases appeared. The organic phase was separated by a pipet and poured into a filter (Whatman phase separator 1 PS silicone treated) that retained water while allowing the organic phase to pass through. The minimum amount of water in the organic phase could interfere with the spectroscopy measurements. The organic solutions were measured in a UV−vis spectrophotometer (JASCO V-670) at 519 nm wavelength. Alternatively, dispersions stirred for 1 week were centrifuged, and the solid material was dried at 80 °C for 12 h before thermogravimetric analysis (TGA). This was performed using Mettler Toledo TGA/SDTA 851e equipment. Samples (about 5 mg) placed in 70 μL alumina pans were heated in an air flow (30 mL (STP)/min) to 900 °C at a heating rate of 20 °C/min. The observed weight loss at temperatures higher than that at which water is removed from the solid (ca. 135 °C) was attributed to the amount of amino acid adsorbed. TGA experiments were preferred instead of the complex UV−vis; however, the weight loss of calcined COMS without adsorption of amino acid was about 1% in the 135−900 °C range. This means that TGA values would be somewhat overestimated.

Figure 1. X-ray diffraction data of the silica materials prepared in this work.

4.0°, and 5.3°) of MCM-41 hexagonal structure, which could be indexed as (100), (110), (200), and (210). In addition, the material retained its XRD order upon calcination. However, an evident contraction was obtained from 2.17 to 1.66 nm (dspacing values obtained from Bragg’s law). The XRD pattern of a blank material prepared without proline is included for comparison, revealing that this material did not possess the MCM-41-type structure, even though several low-angle peaks were observed, they were probably accounted for by a mixture of silica mesophases. The XRD pattern of an extracted sample (noncalcined) is also presented in Figure 1 where the three or four characteristic reflections of MCM-41 could also be indexed to a hexagonal hk0 lattice while the above-mentioned contraction was not so dramatically observed. N2 adsorption−desorption isotherms of silica samples prepared in this work are shown in Figure 2. Calcined L-Pro-

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RESULTS AND DISCUSSION Synthesis of L-Pro-COMS and D-Pro-COMS. C18-TMS (Figure S1a, Supporting Information), besides being a silica source, is a cationic surfactant that also plays the role of a conventional CTA (cetyltrimethylammonium) cation.34 In fact, the charge-matching between organic surfactant and inorganic silicate components established in the case of the hydrothermal synthesis of MCM-41 and SBA-1535 could also be applied to C18-TMS and the amino acid proline. However, as illustrated in Figure S1b, there are important differences between the conventional synthesis of MCM-41 using the CTA cation34 and the syntheses carried out here combining C18-TMS surfactant and proline:12 (a) There is the possibility of condensation between hydrolyzed C18-TMS surfactant molecules to produce dimers. Then the intermolecular van der Waals forces between hydrophobic chains produce stable micelles (van der Waals sphere). (b) There is the possibility of the organization of the aminosilane dimers into micelles where the amino acid molecules and the negatively charged silica species (because of the basic pH) could electrostatically interact with the positively charged C18-TMS dimers (Coulombic sphere). (c) There is a reaction of hydrolyzed C18-TMS surfactant molecules having silanol groups with silicate species via covalent bonds (covalent sphere). This means that the binding energy of the organic−inorganic interphase would be much larger than in the conventional synthesis of MCM-41 where the interaction between silicates and CTA is only electrostatic. The micelle constituted by alternating C18-TMS dimers and negatively charged amino acids or silica species makes possible the efficient transference of chirality to the ordered mesoporous silica in a mechanism where the amino acid leaves its molecular imprint in the silica without being incorporated into the final product.12 The XRD patterns of COMS prepared with D- and L-forms of proline (Figure 1) show four characteristic peaks (at 2.1°, 3.5°,

Figure 2. N2 adsorption−desorption isotherms of the silica materials prepared with L-proline (■), D-proline (●), both calcined, L-proline extracted with HNO3 + H2O2 by microwave irradiation (▲) and calcined without amino acid (×). The inset shows the BJH pore size distribution calculated from the adsorption branch.

COMS and D-Pro-COMS samples possess typical type IV isotherms, which are characteristic of mesoporous materials. The absence of hysteresis is consistent with pore diameters below approximately 4 nm.36 In fact, calcined L- and D-ProCOMS samples possess BJH pore sizes in the 2.6−3.2 nm range with BET specific surface areas as high as 1003 m2/g (sample D-Pro-COMS) (Table 1). The blank material also shows a large BET surface area (1336 m2/g). It is worth 6640

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used). Drying MCM-41 results in little shrinkage and thus would have little influence on porosity. Template removal conditions affected the final textural properties of COMS (pore volume, pore size, and BET specific surface area) but not the symmetry of the pores, as the XRD patterns evidenced. Furthermore, the BJH pore diameter of MCM-41 has been tuned from 1.6 to 4.5 nm using a single surfactant or a mixture of two surfactants with alkyl chains varying from C8 to C22,37,38 while as-synthesized and calcined (at 540 °C) pore diameters estimated from XRD measurements were 3.98 and 3.32 nm, respectively, for MCM-41 sample obtained with C16-CTA+ surfactant having an adsorption diameter of 3.4 nm.39 In addition, it has been argued that the pore size of the assynthesized material increases about 0.225 nm for each increase of one carbon in the surfactant.38 As a consequence, our C18 surfactant chain would be consistent with an expected pore diameter of about 4.1 nm. Figure 3 shows the hexagonal pore distribution of the as-synthesized, calcined, and extracted materials with their correspondent fast Fourier transform (FFT) diffractograms, which are in agreement with the P6mm space group. The transmission electron microscopy (TEM) image of as-made L-Pro-COMS (Figure 3a) is in relative agreement with this statement, showing a pore size of only about 3.5 nm, which can be due to the differences between the surfactant used here (which interacts covalently with silica) and the conventional CTA+. The calcination (Figure 3b) produced an important shrinkage giving rise to TEM pore sizes of 2.3 nm (BJH pore size of 2.6 nm, Table 1), while the chemical extraction (Figure 3c) left behind TEM pore diameters of 3.0 nm (BJH pore size of 4.5 nm, Table 1), closer to those of the as-made material. Proline Adsorption. From Raman spectroscopy, induced circular dichroism (ICD), and TEM, it was shown in a previous publication12 that our calcined COMS is a singularly featured chiral material. In particular, ICD revealed that the handedness of calcined L-Pro-COMS is opposite that of calcined D-ProCOMS, as expected for a pair of enantiomers, while the spectrum of calcined DL-Pro-COMS showed no ICD signal. On the other hand, the TEM characterization showed that the pores in COMS changed their direction, resembling an “eightlike” morphology, which could be related to the Raman and ICD features. These domains would change in orientation in a dimension closer to the molecule diameter than in other helical mesostructured materials exhibiting chirality on a scale about 2 orders of magnitude larger than the mesopore size.35 This would explain the ability of our COMS for enantiomer discrimination.

Table 1. Textural Properties of the Silicas Prepared in this Worka sample blank (calcined) calcined D-Pro-COMS calcined L-Pro-COMS microwave-extracted L-Pro-COMS HNO3 + H2O2 (first) H2SO4, (second) HNO3 + H2O2 two extractions with HNO3 + H2O2

pore size (BJH), nm

surface area (BET), m2/g

pore volume, cm3(STP)/g

template removal, wt %

− 3.2 ± 0.2 2.6 ± 0.2

1336 1003 ± 37 999 ± 29

0.65 0.73 ± 0.02 0.64 ± 0.09

4.4

754

0.96

98.5

4.9

673

0.84

97.8

4.5

728

0.86

98.3

100 100 100

a

BJH parameters are calculated from the adsorption branch of the isotherm. Deviations were calculated from the average of two (calcined D-Pro-COMS) and seven (calcined L-Pro-COMS) analyses. Template removal was calculated from TGA analysis.

mentioning the narrowness of the unimodal pore size distribution (see the inset in Figure 2). As an alternative to calcination, some of the samples were activated by chemical extraction in a MW oven (Table 1). Choosing the appropriate type and concentration of reagents in the digestion process is of vital importance to maximize the efficiency of the organic material oxidation without damaging the inorganic framework. The chemical extractions using H2SO4 destroyed the porous structure. Extraction with HNO3 alone was clearly not sufficient to remove the template entirely, so a mixture of HNO3 and H2O2 was used to improve the method, achieving the highest template removal (98.5%) at a HNO3:H2O2 volume ratio of 1.5:1. For this material, a specific surface area of 754 m2/g was measured, approaching that of the calcined material. The corresponding isotherm and wider (due to the chemical attack that may have dissolved part of the MCM-41 pore walls) BJH pore size distribution are also represented in Figure 2. On the other hand, the double extraction using this mixture did not produce further improvement, i.e., 98.3% template removal and a specific surface area of 728 m2/g. In general, synthesis conditions, drying, and template removal are involved in forming the porosity of porous amorphous silica. In this case, the synthesis conditions were the same for all samples (with the exception of the amino acid form

Figure 3. TEM images of as-made (a), calcined (b), and MW-extracted (c) L-Pro-COMS, the insets being the corresponding Fourier transforms indexed assuming hexagonal symmetry. 6641

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obtained by both techniques are superposed, the agreement was quite good. Many of the adsorption points were repeated 2−4 times, as depicted by the error bands in Figure 4, giving rise to an idea of the reproducibility of the procedures. Figure 4 also displays the calculated isotherms obtained by applying the Langmuir equation (on both sets of UV−vis and TGA values):

Moreover, calcined L-Pro-COMS resolved several racemates: proline (αD/L = 6.3, Table 2), trans-4-hydroxyproline (αD/L = Table 2. Enantiomeric Separation Factors Resulting from Shaking Activated Pro-COMS in Contact with the Racemic Prolinea D/L-racemate L-Pro-COMS

separation factor (αD/L)

calcined MW-extracted: H2O2 + HNO3

6.3 ± 2.0 3.2

MW-extracted: (1) H2SO4, (2) HNO3 + H2O2 MW-extracted: twice with HNO3 + H2O2

1.1

a

3.2

ref

q = qmax

12 this work this work this work

kC 1 + kC

(1)

where q is the amount absorbed (mmol/g), C the concentration in the solution (mmol/L), qmax the maximum adsorption capacity, and k the adsorption equilibrium constant (L/mmol). The fitting gave rise to values of k of 0.036 and 0.097 L/mmol and qmax of 2.85 and 0.68 mmol/g for L- and Dproline adsorption, respectively. The regression coefficient R2 was 0.97 in both cases. The maximum L-proline and D-proline adsorption capacities were ca. 2.3 and 0.6 mmol/g, respectively (Figure 4). As expected, since the amino acid left its imprint in the COMS during synthesis, the solid prepared with L-proline (L-ProCOMS) adsorbed more L- than D-proline. Even though no similar results (proline adsorption on a molecular sieve) have been found in the literature, Gao et al. recently reported the adsorption of glutamic acid, arginine, phenylalanine, leucine, and alanine on several samples of ordered mesoporous material SBA-15 with pore sizes in the 6.5−7.2 nm range.11 The authors measured maximum adsorption capacities in the 0.4−0.7 mmol/g range. Our maximum capacities for L- and D-proline were higher than those reported for SBA-15, which may be due to a stronger interaction between them and the smaller pore size of MCM-41-type COMSs used in this work, with pores of 2.6 ± 0.2 nm (Table 1). The higher adsorption observed for Lproline was a consequence of the chiral nature of the solid used. A final demonstration of the chiral nature of our solids was obtained by performing L- and D-proline separate adsorption at 100 mM on D-Pro-COMS and DL-Pro-COMS, and Figure 5

Determined as in ref 12.

0.70), pipecolic acid (αD/L = 1.5), isoleucine (αD/L = 1.3), valine (αD/L = 1.1), leucine (αD/L = 1.1), and 2-phenylglycine (αD/L = 1.2).12 Similarly, the chemical extraction with HNO3 and H2O2, but not with H2SO4, using microwave irradiation, produced a solid also selective in the separation of DL-proline racemate (αD/L= 3.2, Table 2). Hence, the controlled soft activation of the COMS did not affect its enantioselective performance. The lower separation factor obtained (3.2 vs 6.3) may be due to the fact that the structure of the extracted L-Pro-COMS was slightly damaged upon microwave extraction, as revealed by XRD and N2 adsorption (wider BJH pore size distribution and smaller BET specific surface area than for the calcined material), and has larger pores than the one calcined. In addition, some template remains in the MW-extracted samples (see Table 1), hindering somewhat the access of the amino acid to the chiral centers in the silica. In addition, we demonstrate here that L-Pro-COMS, due to its chiral structure, has different adsorption for L-proline and Dproline enantiomers. Figure 4 depicts the adsorption isotherms

Figure 5. Adsorption of L- and D-proline amino acids (100 mM) on the calcined L-Pro-COMS, D-Pro-COMS, and DL-Pro-COMS. Error bands correspond to the standard deviations of 2−4 experiments.

Figure 4. Adsorption isotherms of L-proline (squares) and D-proline (circles) on calcined L-Pro-COMS, measured by TGA (full symbols) and UV−vis (empty symbols). Lines correspond to the calculated Langmuir isotherms. Error bands correspond to 2−4 experiments performed in the same conditions.

compares the adsorption of both enantiomers on the three possible COMSs. Contrarily to L-Pro-COMS, D-Pro-COMS adsorbed more D- than L-proline while DL-Pro-COMS did not exhibit preferential adsorption. The differences in the maximum amino acid amount adsorbed with L- and D-Pro-COMS (even though the same values would be expected) may be due to the solid preparation (a much higher number of L-Pro-COMS samples were prepared) and amino acid adsorption determi-

of both enantiomers on calcined L-Pro-COMS measured by thermogravimetric and spectrophotometric analyses. TGA determination was the easiest to carry out, giving results over the whole range of proline concentration (0.5−200 mM), while UV−vis spectrophotometry determination was used as corroboration in some cases. In the range where results 6642

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nation error. Finally, proline was used because it is the only proteinogenic amino acid possessing a naturally restricted conformation characterized by the presence of a pyrrolidine ring in which the α-amino group is inserted. Consequently, the chiral center is located in a quite rigid environment, and probably this fact is responsible for the higher enantioselective adsorption of derived COMS, as demonstrated in our previous work.12

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CONCLUSIONS The combination of tetraethyl orthosilicate and quaternized aminosilane (with a templating role) silica sources in the presence of L-proline led to the synthesis of COMSs with structural and textural features similar to those of MCM-41, i.e., a BET specific surface area of about 1000 m2/g and a narrow pore size of 2.6−3.2 nm. As an alternative to calcination, COMSs were also successfully activated by chemical extraction, using a microwave oven, with a combination of H2O2 and HNO3. Both activation strategies (conventional calcination and chemical extraction) yielded materials able to solve proline racemate. Calcined samples exhibited important shrinkage, as noted by XRD and TEM, as compared to as-made and extracted materials. The calcined material (L-Pro-COMS) showed an exceptional performance when the single L- and D-proline enantiomer adsorption was evaluated: L-proline adsorption was clearly above that of D-proline, with maximum adsorption capacities of 2.3 and 0.6 mmol/g, respectively. Interestingly, when both enantiomers contacted D-Pro-COMS (prepared in the presence of D-proline), the adsorption of D-proline was favored over that of L-proline amino acid. These results confirm the chiral nature of L- and D-Pro-COMS due to the imprint left by the amino acid during the preparation of the ordered mesoporous silica, even after calcination.

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ASSOCIATED CONTENT

* Supporting Information S

Figure showing C18-TMS used as the surfactant and a schematic illustration of the three types of interactions between C18-TMS dimers, negatively charged amino acid molecules, and silica species. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 976 762471. Fax: +34 976 761879. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Spanish Ministry of Science and Innovation under Projects MAT2007-61028 and CTQ200800187 and the Aragón Government (Grant GA-LC-019/2011) is gratefully acknowledged. C.C. also acknowledges a “Juan de la Cierva” grant from the Spanish Science and Innovation Ministry.

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